The subject invention generally concerns the field of probing a device under test, and specifically concerns probing a connection point of a ball grid array (BGA) circuit, or a conductor of a closely-spaced array of conductors (i.e., signal lines).
A ball grid array (BGA) device is an integrated circuit having multiple contact points typically arranged in a 50-mil ball-spacing grid formation beneath the body of the integrated circuit. Heretofore, in order to monitor a signal at a particular connection point of a BGA device, it was necessary to route the signals beyond the perimeter of the device via printed circuit traces to reach a particular isolation resistor placed as closely as possible to the BGA device. Unfortunately, these printed circuit traces often traverse a relatively long and circuitous path on their way from the connection point of the BGA to an isolation resistor at the periphery of the BGA package. These less-than-optimum signal paths adversely affect the signal being monitored by causing unacceptable ringing, overshoot, and a plateau close to the reference voltage. The same problem occurs when one is trying to probe conductors of a array of closely-spaced conductors. What is needed is a better solution to the problem of probing BGA devices or closely-spaced conductors that avoids the above-identified problems.
In a first aspect of the invention, a method and apparatus for probing a terminal of a ball grid array device, or a conductor of an array of closely-spaced conductors, preferably employs a buried tip resistor located substantially adjacent to the point to be monitored. In this way, a relatively short stub is provided from the connection point to the tip resistor.
In a second aspect of the invention, a receiver amplifier arrangement substantially eliminates an offset error that is exhibited in prior art amplifier arrangements, the offset error being introduced into the signal to be measured due to variation in the resistance value of tip resistor.
In a third aspect of the invention, it is recognized that buried resistors are not an essential element of the subject invention, but rather, the subject invention is applicable in probing situations involving the use of tip resistors having a tolerance range greater than 1%.